This invention relates to a magnetic recording medium such as magnetic tape, magnetic sheet, magnetic disc, etc.
In general, a magnetic recording medium is prepared by applying a magnetic paint containing magnetic powder, a binder resin, etc. on a support and drying the coating. In the magnetic recording medium thus prepared, the magnetic layer comes into vigorous sliding contact with a magnetic head to give rise readily to abrasion or powder drop-off, whereby lowering in reproduction output, output fluctuation, drop-out, increase of noise are liable to be caused.
For overcoming such drawbacks, countermeasures have been taken to enhance mechanical strength and durability of the magnetic layer side by provision of an overcoat layer (hereinafter sometimes referred to as OC layer) on the magnetic layer. Also, in some cases, for the purpose of improving running performance or winding appearance stability (in the case of tape), a back-coating layer (hereinafter sometimes referred to as BC layer) may be provided on the support on the side opposite to the magnetic layer.
For example, concerning BC layer, it is well known in the art to use a urethane resin as the binder resin, as disclosed in Japanese Patent Publications Nos. 2613/1965, 15004/1963 and 29769/1982. Urethane resins known in the art are synthesized from a polymeric diol, a diisocyanate, a chain elongating agent and (optionally) a cross-linking agent. As the polymeric diol, there may be included polyester diols obtained from adipic acid, butane diol, etc., polyether diols, polycarbonate diols and the like; and the diisocyanate available may be, for example, diphenylmethane diisocyanate, etc. The chain elongating agent may comprise ethylene glycol, butane diol and the like, and the cross-linking agent may be, for example, a polyol, a polyamine, etc.
However, such urethane resins in general may be excellent in flexibility, but they are lacking in hardness. Therefore, in sliding contact against a guide pin or a magnetic head, the magnetic recording medium is poor in mechanical strength and also involves problems in aspects of running performance or powder drop-off.
On the other hand, for the magnetic powder, ferromagnetic powder is generally employed, which has a large magnetic moment, and interactions will readily occur between the particles to cause particle agglomeration, whereby it is impossible to disperse particles uniformly in the binder resin. As the result, the magnetic recording medium becomes markedly inferior not only in the magnetic characteristics such as rectangular ratio, etc. but also inferior in properties such as surface characteristic, running performance, abrasion resistance, etc. whereby powder drop-off, drop-out will occur to bring about lowering in reproduction output and still characteristic.
As the magnetic powder for such a magnetic layer, .gamma.-Fe.sub.2 O.sub.3 is generally employed. The .gamma.-Fe.sub.2 O.sub.3 powder ordinarily consists of co-existing trivalent iron (Fe.sup.3+) and divalent ion (Fe.sup.2+), mostly of Fe.sup.3+ with a proportion of Fe.sup.2+ being 2 to 3 atomic %. The magnetic characteristics are considered to be better as the Fe.sup.3+ is greater in amount, and therefore Fe.sup.2+ is made as small as possible.
Also, in some kinds of magnetic recording medium, for example, a magnetic tape for a video recording, there is the system in which the light transmittance of the magnetic layer is made smaller (shielding force is made greater) and the difference in light transmittance from the reader tape portion is detected to control running of the tape. In this case, the shielding force is increased by addition of carbon black in the magnetic layer to suppress light transmittance to 0.05% or less. However, by addition of carbon black for the purpose of increasing the shielding force, the magnetic layer will become weaker in strength to be poor in durability.
For the purpose of removing such drawbacks, (1) Japanese Patent Publication No. 44967/1982 proposes to use a magnetic paint containing a polyvinyl butyral resin and a polyfunctional isocyanate compound as the binder components of the magnetic powder. Also, in (2) Japanese Patent Publication No. 56128/1982, it has been proposed to use as the binder resin for the magnetic layer a composition comprising a vinyl chloride-vinyl acetate-vinyl alcohol copolymer with a molecular weight distribution of 1.80 to 2.40 and a urethane polymer as the main components, to which a low molecular weight polyisocyanate is added.
However, dispersibility of the magnetic paints obtained by the above known techniques (1) and (2) cannot necessarily be said to be satisfactory. Besides, due to greater paint viscosity and low durability and abrasion resistance of the coating (magnetic layer), it is not possible to reduce clogging of the head due to powder drop-off or generation of drop-out. Generally speaking, a magnetic recording medium is required to satisfy the following requirements in recording and reproduction:
(1) Since the medium runs while being in contact with magnetic head, guiding members, etc., the magnetic layer is required to be excellent in running performance with a small coefficient of friction and also small in abrasion of the magnetic layer with excellent durability, because it comes into vigorous sliding contact with a magnetic head during reproduction; and
(2) the magnetic characteristics or electromagnetic converting characteristics are influenced chiefly by the dispersed state of the magnetic powder.
For the above requirement (1), namely improvement of lubricity and abrasion resistance, various lubricants have been attempted in the prior art to be contained in the magnetic layer, and silicone oil with great lubricating effect has been practiced to be contained in the magnetic layer.
However, while silicone oil is excellent in lubricity, it is poor in compatibility with a binder resin and therefore involved the drawbacks such as formation of pinholes through ununiformization of the magnetic layer or blooming onto the surface of the magnetic layer, although it could sufficiently reduce the frictional resistance of the magnetic layer.
For removing such drawbacks, in the prior art, there have been known techniques to add into the magnetic layer an alcohol-modified organic silicone compound as disclosed in Japanese Provisional Patent Publication No. 37731/1982, a carboxyl-modified organic silicone compound as disclosed in Japanese Provisional Patent Publication No. 37732/1982, an amino-modified organic silicone compound as disclosed in Japanese Provisional Patent Publication No. 37733/1982 or an organic silicone compound as disclosed in Japanese Provisional Patent Publication No. 37734/1982. However, according to these known techniques, although lubricating effect may be improved, electromagnetic converting characteristics of the magnetic recording medium are not satisfactory.
For improvement of abrasion resistance of the magnetic layer, it has been practiced to add an abrasive material such as alumina, etc. However, because the urethane resin as the binder resin is soft as mentioned above, the durability of the magnetic layer becomes worsened unless particle sizes of the abrasive particles are made to be relatively larger. For example, good abrasive force can be exhibited if the abrasive particles are made to have particle sizes of 0.5 .mu.m or larger, but contrariwise there is involved the drwback that the magnetic head in contact with the magnetic layer will readily be damaged.
Also, as the curing agent for curing the magnetic layer, an isocyanate has been known, but it has hitherto been recognized that the amount of this curing agent cannot be increased so much. That is, if the amount of curing agent is too much, the magnetic layer will be embrittled to form readily cracks, etc. This is particularly marked when a conventional binder resin is employed in the magnetic layer, and for this reason the amount of the curing agent is described as 10% by weight or less of the total amount of the binder resin and the curing agent (see Japanese Patent Publications Nos. 15046/1981 and 41484/1979).
For improvement of the above requirement (2), namely dispersed state of magnetic powder, various dispersing agents have been attempted in the prior art to be added in the magnetic layer.
For example, addition of a dialkylsulfosuccinic acid salt is known to act effectively as a dispersing agent to improve filling degree and orientatability of the magnetic powder and impart excellent magnetic characteristics or electromagnetic converting characteristics thereto. When such a dispersing agent is employed, the binding force between the magnetic powder and the binder resin may be lowered, or blooming of the dispersing agent may readily occur on the magnetic layer surface, whereby various phenomena such as powder drop-off, output fluctuation or tape squeaking may be caused to make durability inferior. Also, a polyalkylene oxide alkylphosphate can also act effectively as the dispersing agent similarly as the above dispersing agent, but also involves the same drawbacks.
For this reason, it has been thougt of to employ a dialkylsulfosuccinic acid salt and a fatty acid-modified silicone compound in combination, as disclosed in Japanese Provisional Patent Publication Nos. 152535/1982 and 152537/1982. However, even in these known techniques, it can yet hardly be said that running performance, durability and electromagnetic converting characteristics are satisfactory.
On the other hand, the solvent for the magnetic paint to be used for formation of a magnetic layer will influence the dispersibility of the magnetic powder, and hence the recording and reproduction characteristics, and yet itself required to be stable and have a long pot life. However, such solvents have hitherto not necessarily been studied thoroughly.